A desirable type of photodetector is a two-color infrared radiation (IR) detector having simultaneous sensitivity in two spectral bands. The spectral bands may include short wavelength IR (SWIR), medium wavelength IR (MWIR), long wavelength IR (LWIR), and very long wavelength IR (VLWIR). An array of two-color IR detectors may be employed in a number of imaging applications wherein it is required to simultaneously detect radiation within two spectral bands from a scene within a field of view of the array. By example, the array may simultaneously detect LWIR and MWIR, or LWIR and SWIR.
It is desirable to employ Liquid Phase Epitaxy (LPE) as a semiconductor layer epitaxial growth technique in that LPE provides high quality pn-junctions. However, it is difficult to grow, by LPE, a LWIR layer over an existing MWIR layer, in that the LPE melt for the LWIR material tends to dissolve the narrower bandgap MWIR layer. As a result, Molecular Beam Epitaxy (MBE) or Metal Organic Chemical Vapor Deposition (MOCVD) is typically employed to fabricate a LWIR layer upon a MWIR layer. However, the resulting pn-junction quality of the MBE or MOCVD grown LWIR/MWIR interface is, at present, inferior to that which would be obtained if LPE were used to form the LWIR/MWIR interface.
In U.S. Pat. No. 4,847,489, Jul. 11, 1989, entitled "Light Sensitive Superlattice Detector Arrangement with Spectral Sensitivity" Dietrich discloses a detector arrangement comprising a plurality of photosensitive detector elements. Each of the detector elements has a multilayer structure of alternating positively and negatively doped photosensitive semiconductor material having a superlattice structure. A control voltage is said to control spectral light sensitivity, and an optical filter arrangement is provided for dividing the photodetectors into an upper and lower effective spectral range group.
In U.S. Pat. No. 4,753,684, Jun. 28, 1988, "Photovoltaic Heterojunction Structures" Ondris et al. describe a three layer, double heterojunction Group II-VI photovoltaic structure.
In Japanese Patent No. 55-101832, Aug. 4, 1980, Makoto Itou discloses, in the Abstract, an infrared detector comprised of n-type HgCdTe having electrodes 2 and 3 arranged on opposite surfaces. A polarity of a bias voltage is switchably coupled to the electrodes 2 and 3. This device is said to enable rays of wide wavelength ranges to be detected by only one semiconductor detector.
General information regarding this subject matter is found in an article entitled "HgCdTe and Related Alloys" D. Long and J. L. Schmit, Semiconductors and Semimetals, Vol. 5, IR Detectors, Academic Press 1970.
An article entitled "Some Properties of Photovoltaic Cd.sub.x Hg.sub.1-x Te Detectors for Infrared Radiation", by J. M. Pawlikowski and P. Becla, Infrared Physics, Vol. 15 (1975) pp. 331-337 describes photovoltaic p-n junction detectors constructed of HgCdTe crystals and epitaxial films. The authors report that the position of a photosensitivity maximum is shifted within a spectral region of 1-9 microns by changing a molar fraction of cadmium.
It is an object of this invention to provide a method of fabricating a two-color infrared radiation detector by an LPE process, the detector including an LPE grown LWIR layer formed adjacent to a MWIR or a SWIR layer.
It is a further object of the invention to provide an array comprised of a plurality of three-layer n-p-n or p-n-p photodetector structures wherein at least a LWIR responsive layer is formed by LPE, and in which a middle, electrically common layer is electrically and physically continuous across the array.